It is known that a medium intensity magnetic separator (MIMS) is designed to attract and remove ferromagnetic material by means of a drum that generates a magnetic induction in a range of about 200-700 mT depending on the distance from the drum and the magnets employed, which are usually made of a neodymium-iron-boron alloy (Nd—Fe—B).
A MIMS can be used as a protection device for a wet high intensity magnetic separator (WHIMS), or as an autonomous magnetic separator for the concentration of metalliferous minerals with low magnetic susceptibility such as franklinite, ilmenite, some type of hematite, etc.
The conventional structure of a MIMS drum provides for a cylinder of ferromagnetic steel with high magnetic permeability on the outside of which longitudinal grooves are formed to house permanent magnets arranged in longitudinal rows of alternate N-S polarity, i.e. each row includes a single polarity that is different from those of the two adjacent rows (e.g. U.S. Pat. No. 5,636,748). These rows of magnets extend over a limited arc of the drum, typically about 130°-160°, with a final zone (in the direction of rotation of drum) for releasing the ferromagnetic material in which the magnets are progressively tapered to be farther from the external surface of the cylinder. The cylinder is then enclosed by a shell of non-magnetic material that rotates around it drawing the material (mineral or else) containing the ferromagnetic particles to be separated.
The size of the magnets used in prior art drums is quite large and usually increases with the drum diameter, whereby the number of longitudinal rows arranged along the magnetic arc is rather limited and the distance between the cylinder and the shell cannot drop below a given threshold taking into account the drum manufacturing tolerances and the size of the magnets. These geometrical characteristics of known drums negatively affect their performance due to two kinds of drawbacks.
A first serious drawback stems from the fact that since the different polarities are rather far from each other the length of the magnetic dipole (equal to the distance between two polarities with the same sign) results quite great, with values in the order of 200-400 mm like in the case of MIMS marketed by Mineral Technologies Pty. Ltd. (Carrara, Qld—Australia) and by Longi Magnet Co. Ltd. (Fushun—China). As a consequence, the values of the magnetic field on the active surface of the MIMS have a strongly sinusoidal pattern, with nominal values only at the polarities and close thereto (therefore over not more than 30% of the magnetic arc) whereas in the rest of the magnetic arc the values are significantly lower.
A second drawback that further reduces the effectiveness of known drums is the play between the cylinder and the shell which in the above-mentioned MIMS available on the market is of at least 5 mm, to which the shell thickness ranging between 1.5 and 5 mm must be added, whereby the minimum distance between the magnet active surface and the treated material is in the order of 7-8 mm.